1. Technical Field of the Invention
This invention relates to a change speed apparatus for a tractor. More particularly, the invention relates to a change speed apparatus for a tractor having a main change speed device with a combination of gear trains mounted on a first shaft and a second shaft extending parallel to each other for producing a plurality of speeds on an output shaft, and a backward and forward drive switching device cooperable with the main change speed device to produce forward drive and backward drive in the same number of speeds.
2. Prior Art of the Invention
When carrying out a front loading operation with a tractor, switching between forward driving and backward driving has to be made frequently. To perform this operation efficiently, it is desirable that the tractor is driven forward and backward substantially at the same speed. A known change speed apparatus for a tractor that meets the above requirement includes a backward and forward drive switching device as described in Japanese Patent Laying-Open Publication H10-6792 or H11-78560, for example.
The conventional change speed apparatus for a tractor includes a main change speed device disposed in a transmission system for transmitting power from an engine to wheels. The main change speed device produces a plurality of drive speeds. A backward and forward drive switching device is disposed at an input side (upstream side) or an output side (downstream side) of the main change speed device to provide forward drive and backward drive at the same speed.
Where, as shown in FIG. 8, for example, a backward and forward drive switching device 152 is disposed at an input side of a main change speed device 151, the power of an engine 153 is inputted to the backward and forward drive switching device 152 through a main clutch 154, to be switched to forward drive or backward drive. The drive is then inputted to an input shaft 155 of main change speed device 151 to be changed to one of four speeds, and outputted to an output shaft 156.
That is, the drive, whether forward or backward, is inputted to the main change speed device 151 through the same input shaft 155. For this purpose, the backward and forward drive switching device 152 includes a pair of gear trains 158 and 159 disposed forwardly and rearwardly of a backward and forward drive switching clutch 157. One of the gear trains is a reversing gear train 158 for providing backward rotation, and the other a back return gear train 159 for returning the backward rotation to the input shaft 155 of main change speed device 151.
The same concept of one input shaft and one output shaft has been applied also to the construction having the backward and forward drive switching device disposed at the output side (downstream side) of the main change speed device. That is, to provide backward drive, power received from the output shaft of the main change speed device is reversed by a reversing gear train disposed forwardly of a backward and forward drive switching clutch, and thereafter returned through a back return gear train disposed rearward of the drive switching clutch, to the same shaft used for transmitting forward drive.
However, each of the conventional change speed apparatus noted above has an increased fore and aft length since the backward and forward drive switching device includes two gear trains, i.e. the reversing gear train and return gear train, forwardly and rearwardly of the switching clutch. The increased fore and aft length of the change speed apparatus results in an increased distance between front wheels and rear wheels. This impairs the small turn performance of the tractor.
A primary object of this invention, therefore, is to provide a change speed apparatus for a tractor which has a reduced fore and aft length.
The above object is fulfilled, according to this invention, by a change speed apparatus for a tractor wherein, between the main change speed device and the backward and forward drive switching device, the forward drive is transmitted through the first shaft and the backward drive is transmitted through the second shaft.
By employing this construction, the conventional backward return gear train may be dispensed with. This not only reduces the number of components but reduces the fore and aft length of the change speed apparatus. The fore and aft length of the transmission case may be reduced correspondingly, which in turn reduces the wheelbase to improve the small turn performance of the tractor.
In this way, this invention provides a compact change speed apparatus for a tractor.
Where the backward and forward drive switching device is disposed upstream of the main change speed device, the forward drive is transmitted from the backward and forward drive switching device to the main change speed device through the first shaft, and the backward drive is transmitted from the backward and forward drive switching device to the main change speed device through the second shaft.
Some conventional tractors have no backward and forward drive switching device. The tractors with and those without the backward and forward drive switching device are regarded as different in specification and design, and thus as different models. This complicates control and management in designing, manufacture and maintenance. According to this invention, the backward and forward drive switching device and the main change speed device are disposed in a transmission case, the backward and forward drive switching device is pre-assembled in a gear case fixable to the transmission case. With this construction, only by assembling to the transmission case the gear case pre-assembled with the backward and forward drive switching device, a tractor without the backward and forward drive switching device may advantageously be converted into one having the backward and forward drive switching device.
According to this invention, the transmission case may be divided by a partition into a clutch chamber for housing a main clutch for connecting and disconnecting power from an engine, and a transmission chamber for housing the main change speed device, the gear case being disposed in the clutch chamber, the main clutch being operable by a release fork disposed between the main clutch and the gear case, the release fork being curved to bulge toward the main clutch. This construction reduces the distance between the main clutch and gear case, to make the change speed apparatus compact.
A conventional backward and forward drive switching device is disposed in the transmission chamber storing a lubricant for the transmission case, is therefore remote from a control lever disposed adjacent the steering wheel. It has been difficult to connect the switching device and the control lever, and a complicated connecting mechanism has been required. It is conceivable to place the backward and forward drive switching device in the clutch chamber close to the control lever. However, the clutch chamber is a dry chamber, and it is difficult to place, in the clutch chamber, the backward and forward drive switching device which needs lubrication. In this invention, the backward and forward drive switching device and the main change speed device may be disposed in a transmission case, and the transmission case is divided by a partition into a clutch chamber for housing a main clutch for connecting and disconnecting power from an engine, and a transmission chamber for housing the main change speed device, the backward and forward drive switching device having a control lever thereof disposed on a steering wheel unit, the control lever being interlocked to the backward and forward drive switching device through an operating shaft extending vertically in the clutch chamber. By employing such a construction, the control lever and the operating shaft are disposed close to each other, and may advantageously be interconnected by a simple connecting mechanism.
Other features, functions, effects and advantages of the present invention will be appreciated upon reading the following description with reference to the drawings.